U.S. patent number 4,143,521 [Application Number 05/766,757] was granted by the patent office on 1979-03-13 for process for the production of ethylene.
This patent grant is currently assigned to Stone & Webster Engineering Corporation. Invention is credited to Sharad S. Gandbhir, Benjamin V. Pano.
United States Patent |
4,143,521 |
Pano , et al. |
March 13, 1979 |
Process for the production of ethylene
Abstract
A process for the production of ethylene wherein low-level waste
heat released during ethylene production is utilized in an ammonia
absorption refrigeration system to generate a refrigerant, which
refrigerant is used for cooling various process streams thereby
reducing the energy requirements of the process.
Inventors: |
Pano; Benjamin V. (Watertown,
MA), Gandbhir; Sharad S. (Newton, MA) |
Assignee: |
Stone & Webster Engineering
Corporation (Boston, MA)
|
Family
ID: |
25077441 |
Appl.
No.: |
05/766,757 |
Filed: |
February 8, 1977 |
Current U.S.
Class: |
62/101; 585/650;
585/634 |
Current CPC
Class: |
B01J
19/0006 (20130101); B01J 19/0013 (20130101); F25B
15/04 (20130101); C10G 9/002 (20130101); B01J
2219/00159 (20130101); Y02P 30/464 (20151101); C10G
2400/20 (20130101); B01J 2219/00162 (20130101); Y02P
30/40 (20151101); B01J 2219/00103 (20130101) |
Current International
Class: |
B01J
19/00 (20060101); F25B 15/02 (20060101); F25B
15/04 (20060101); C10G 9/00 (20060101); F25B
015/00 (); C07C 003/00 (); C07C 005/00 (); C07C
011/02 () |
Field of
Search: |
;62/101,476
;260/683R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: King; Lloyd L.
Attorney, Agent or Firm: Morgan, Finnegan, Pine, Foley &
Lee
Claims
We claim:
1. In a process for thermally cracking hydrocarbons to produce
olefins said process having a cracking zone, a rapid cooling zone,
a primary fractionation and cooling zone, a compression zone and a
purification zone wherein waste heat is released and cooling of
system streams is effected, the improvement comprising the steps
of:
heating a rich ammonia stream from an ammonia absorption
refrigeration system with waste heat released from quench water in
said primary fractionation and cooling zone and with low-level
waste heat released from steam emanating from steam turbines in
said compression zone, to generate an ammonia refrigerant in said
ammonia absorption refrigeration system; and
cooling said system streams in said olefin production process with
ammonia refrigerant generated in said ammonia absorption
refrigeration system.
2. The process of claim 1 wherein said waste heat released from
said quench water is heat released from quench water exiting a
direct contact cooler in said primary fractionation and cooling
zone.
3. The process in claim 1 wherein said system streams are cooled by
said ammonia refrigerant prior to introducing said system streams
into compressors utilized to compress said system streams.
4. The process in claim 1 wherein cooling is conducted on overhead
vapors exiting a propane-propylene fractionation column.
5. In a process for the production of ethylene by cracking a
hydrocarbon to produce a process stream of cracked gases,
introducing said process stream of cracked gases into a rapid
cooling zone to cool said gases, passing said cooled process stream
of cracked gases into a primary fractionation and cooling zone to
further cool said gases and remove heavy hydrocarbons, passing said
process stream of cracked gases into a compression zone and,
thereafter, directing said process stream of compressed cracked
gases through a purification zone wherein overhead vapors are
generated and ethylene is recovered from said process stream of
cracked gases, the improvement which utilizes low-level waste heat
released in said primary fractionation and cooling zone and said
compression zone to generate an ammonia refrigerant for cooling
said process stream of cracked gases in said compression zone and
said overhead vapors in said purification zone comprising:
(i) heating a rich ammonia stream with low-level waste heat
released from quench water in said primary fractionation and
cooling zone and with low-level waste heat released from steam
emanating from steam turbines in said compression zone;
(ii) separating the heated rich ammonia stream into substantially
pure ammonia vapor and a lean ammonia stream;
(iii) condensing said substantially pure ammonia vapor to ammonia
liquid;
(iv) passing said ammonia liquid through pressure reducing means to
produce said ammonia refrigerant;
(v) cooling said process stream of cracked gases in said
compression zone and said overhead vapors in said purification zone
with said ammonia refrigerant;
(vi) thereafter, combining said ammonia refrigerant with said lean
ammonia stream to form a rich ammonia stream; and
(vii) directing said rich ammonia stream back to step (i).
6. The process of claim 5 wherein said rich ammonia stream is
heated in step (i) with low-level waste heat released in said
purification zone.
7. The process of claim 6 wherein the source of said low-level
waste heat in said purification zone is a propylene refrigerant
system.
8. The process of claim 5 wherein said rich ammonia stream is
heated in step (i) to a temperature of about 130.degree. to
200.degree. F.
9. The process of claim 5 wherein said ammonia refrigerant is
produced in step (iv) at a temperature of about 55.degree. to
70.degree. F.
10. The process of claim 9 wherein said ammonia refrigerant
produced in step (iv) is used as a cooling medium in said propylene
refrigerant system.
11. A process according to claim 5 wherein in step (i) the
temperature of said rich ammonia stream entering said primary
fractionation and cooling zone and said compression zone is about
95.degree. F.
12. A process according to claim 5 wherein said ammonia liquid in
step (iv) enters said pressure reducing means at a pressure of
about 200 p.s.i.a and leaves said pressure reducing means at a
pressure of about 100 to 130 p.s.i.a.
13. A process according to claim 5 wherein in step (v) said ammonia
refrigerant cools said process stream of cracked gases in said
compression zone to a temperature of about 65.degree. to 85.degree.
F.
14. A process according to claim 5 wherein in step (v) said ammonia
refrigerant cools said overhead vapors in said purification zone to
a temperature of about 63.degree. to 85.degree. F.
15. A process according to claim 5 wherein in step (vi) the
concentration of said rich ammonia stream is about 70% by weight
ammonia.
16. A process according to claim 5 wherein said primary
fractionation and cooling zone includes a direct contact cooler and
wherein said rich ammonia stream is heated in step (i) with quench
water from said direct contact cooler.
17. A process according to claim 16 wherein said purification zone
includes a propane-propylene fractionation column and wherein
reboiler heat is supplied for said propane-propylene fractionation
column by said quench water from said direct contact cooler.
18. A process according to claim 5 wherein said steam from said
steam turbines is at a pressure of about 5 p.s.i.a.
Description
FIELD OF THE INVENTION
The present invention relates to the production of ethylene.
In one of its aspects, the invention relates to the use of
low-level waste heat released during ethylene production to
generate refrigerant in an ammonia absorption refrigeration
system.
In a more specific aspect, the present invention relates to the use
of low-level waste heat released during ethylene production to
generate refrigerant in an ammonia refrigeration system which
refrigerant is used for cooling process streams during ethylene
production.
BACKGROUND OF THE INVENTION
1. Description of the Prior Art
As is well known to those skilled in the art, conventional ethylene
manufacturing facilities utilize cooling water, which is generally
available at a temperature of about 80.degree. F., to cool various
equipment and process streams during various stages of ethylene
production. Cooling water is utilized as the cooling medium in the
primary frationation and cooling zone to cool the hot quench water,
in the compression zone to condense the steam used to drive steam
turbines which power cracked gas compressors and to cool the
compressor cracked gases. It is also used in the purification zone
to cool overhead vapors in the condensers associated with
fractionation towers and in the propylene refrigeration system,
which is a part of the purification zone, to condense both the
propylene vapor which has been compressed and the steam used to
drive turbines which power the propylene compressors.
Some of the equipment or apparatus described above develop low
levels of heat which is either rejected to cooling water or to the
atmosphere. For example, the low-level waste heat which is
generated in the turbine exhaust of a steam gas turbine compressor
is, under conventional techniques, rejected to either cooling water
or the atmosphere. Heretofore, the rejection of available low-level
heat generated in the various equipment as explained previously
was, from an economical standpoint, not entirely unobjectionable
since fuel was relatively inexpensive and readily available.
Unfortunately, however, the relatively high cost of today's fuels,
coupled with present efforts to conserve energy, now makes it
necessary that new procedures be developed so as to minimize the
amount of fuel required for an ethylene production system and, if
possible, to capture sources of energy such as is present in
low-level waste heat for possible use in the ethylene production
plant.
Attempts to utilize low-level waste heat in ethylene production
have not heretofore been entirely satisfactory due primarily to the
fact that the amount and temperature of the heat generated were not
sufficient to utilize the heat as a heating source for the
equipment, which in some cases requires temperatures sufficient to
convert water to steam. As mentioned previously, cooling water has
been the cooling medium utilized to cool equipment and process
streams. An ammonia absorption refrigeration system has already
been proposed as a possible technique for cooling certain equipment
in oil refineries and elsewhere such as is shown in Refiner &
Natural Gasoline Manufacture, Vol. 20, No. 5, May, 1941, page (146)
56 and U.S. Pat. No. 3,817,050 (issued June 18, 1974). However, no
satisfactory system has heretofore been proposed for supplying the
energy required for generating the ammonia refrigerant.
It has been found that the low-level waste heat produced during
ethylene production can be effectively utilized to generate
refrigerant in an ammonia absorption system used for cooling
equipment and process streams. Advantageously, no additional
outside sources of heat are required to generate the ammonia
refrigerant and, since cooling of the process streams is effected
at temperatures significantly lower than could be obtained using
cooling water, expensive process equipment can be either eliminated
or reduced in size and the overall cost of producing ethylene can
be significantly reduced.
It is therefore an object of the present invention to provide a
cooling method for cooling process streams produced during ethylene
manufacture which method utilizes low-level waste heat, e.g., the
heat of hot water or low-pressure steam, to generate refrigerant in
an ammonia absorption refrigeration system.
Another object is to utilize a refrigeration method in ethylene
production which method permits lower operating temperatures and
pressures in the product recovery purification zone.
A further object is to provide a refrigeration system which can be
readily integrated into the overall ethylene production process to
achieve the more economical production of ethylene.
These and other objects will become apparent from the following
summary of the invention and description taken in conjunction with
the accompanying drawing.
SUMMARY OF THE INVENTION
Broadly contemplated, the present invention provides an improvement
in a process of thermally cracking hydrocarbons to produce olefins
wherein waste heat is released and cooling of system streams is
effected, the improvement comprising the steps of:
heating a rich ammonia stream from an ammonia absorption
refrigeration system with waste heat released by said olefin
production process to generate an ammonia refrigerant in said
ammonia absorption refrigeration system; and
cooling said system streams in said olefin production process with
ammonia refrigerant generated in said ammonia absorption
refrigeration system.
According to the broad concept of the invention, the source of heat
released by the olefin production process can be heat released by
quench water from a direct contact cooler. In addition, heat
released by steam turbines operating compressors utilized to
compress the system streams can also be a source of heat. Moreover,
according to the broad concept of the invention, system streams can
be cooled with generated ammonia refrigerant at various points,
e.g., prior to introducing the system streams into the compressors
utilized for compressing the system streams.
In a more specific aspect the present invention provides an
improvement in the process for the production of ethylene wherein a
hydrocarbon is cracked to produce a process stream of cracked
gases. The process stream of cracked gases is introduced into a
rapid cooling zone to cool the gases and is, thereafter, passed
into a primary fractionation and cooling zone to further cool the
gases and remove heavy hydrocarbons. The process stream of cracked
gases is then passed into a compression zone and is, thereafter,
directed through a purification zone wherein overhead vapors are
generated and ethylene is recovered from the process stream of
cracked gases. The improvement of the present invention utilizes
low-level waste heat released in the primary fractionation and
cooling zone and the compression zone to generate an ammonia
refrigerant for cooling the cracked gas process stream in the
compression zone and the overhead vapors in the purification zone.
More specifically, the improvement comprises:
(i) heating a rich ammonia stream with low-level waste heat
released in said primary fractionation and cooling zone and said
compression zone;
(ii) separating the heated rich ammonia stream into substantially
pure ammonia vapor and a lean ammonia stream;
(iii) condensing said substantially pure ammonia vapor to ammonia
liquid;
(iv) passing said ammonia liquid through pressure reducing means to
produce said ammonia refrigerant;
(v) cooling said process stream of cracked gases in said
compression zone and said overhead vapors in said purification zone
with said ammonia refrigerant;
(vi) thereafter, combining said ammonia refrigerant with said lean
ammonia stream to form a rich ammonia stream; and
(vii) directing said rich ammonia stream back to step (i).
The improvement additionally comprises condensing propylene
refrigerant vapors in the propylene refrigeration system of the
purification zone with said ammonia refrigerant.
DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic diagram illustrating the sequence of
treatment zones for producing ethylene and their relationship to
the ammonia absorption refrigeration system.
FIG. 2 is one arrangement of some of the apparatus parts
illustrating pertinent flow characteristics within the primary
fractionation and cooling zone of FIG. 1.
FIG. 3 is an illustration of one of the compression systems or
stages illustrating some of the apparatus parts and pertinent flow
characteristics within the compression zone of FIG. 1.
FIG. 4 is a preferred arrangement of some of the apparatus parts of
the propane-propylene fractionation column in the purification zone
and also illustrating pertinent flow characteristics within the
purification zone of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1, the sequence of treatment steps (shown as
zones) for producing ethylene includes: a cracking zone generally
represented by reference numeral 2, a rapid cooling zone 4, a
primary fractionation and cooling zone 6, a compression zone 8, and
a purification zone 10. The ammonia absorption refrigeration system
which is used to treat process streams in the pertinent treatment
zones is generally illustrated by reference numeral 12. The
hydrocarbon feed is initially directed through line 14 into the
cracking zone 2 of an ethylene processing plant. This feed stock
can comprise ethane, propane, butane, pentane, naphtha, gas oil or
mixtures of these hydrocarbons.
The cracking zone 2 includes a cracking furnace (not shown) which
receives the hydrocarbon feed which at this stage can be in the
liquid or vapor phase or mixed liquid vapor phase. The hydrocarbon
feed is heated in the cracking furnace at highly elevated
temperatures either by superheated steam, radiant heat, convection
heat or a combination thereof to achieve the desired cracking. The
procedures for cracking the aforementioned hydrocarbons are well
known to those skilled in the art.
The hydrocarbon gases leaving the cracking zone at high
temperatures are subjected to immediate cooling to terminate the
pyrolysis reaction and ensure the production of a cracked product
having a high ethylene content. This cooling can be effected by
introducing the process stream of gases into a rapid cooling zone
4. Thus, the hydrocarbon gases exiting the cracking zone 2 are
introduced through line 16 into rapid cooling zone 4.
The apparatus and equipment in the rapid cooling zone 4 are
conventional and include cooling means which are adapted to rapidly
cool hot fluids. A cooling apparatus and process which can be
utilized as the cooling means in the rapid cooling zone 4 is
illustrated in U.S. Pat. No. 3,403,722 (issued Oct. 1, 1968).
The cooled hydrocarbon gases (generally cooled to a temperature of
about 1000.degree. to 1400.degree. F.) exit the rapid cooling zone
4 through line 18 and are introduced into a fractionation tower 20,
seen in FIG. 2, which is located in the primary fractionation and
cooling zone 6 wherein the hydrocarbon gases are further cooled and
subjected to primary fractionation to remove a fuel oil fraction
from the cracked gas stream. Thus, referring to FIG. 2, it will be
seen that the cooled hydrocarbon gases enter the primary
fractionation and cooling zone 6 and enter a conventional primary
fractionation tower 20 through line 18. The fractionation tower 20
has a direct contact cooler 22 associated therewith. The primary
fractionation tower 20 is conventional and in general is a low
pressure fractionator which separates a fuel oil fraction from the
cracked gas stream.
The fuel oil fraction (having a molecular weight of from about 170
to about 200) in the primary fractionation tower 20 exits the
primary fractionation tower 20 through line 24 and is subsequently
recovered.
The remainder of the cracked gas streams (comprising H.sub.2 and
C.sub.1 to C.sub.12 hydrocarbons) is passed into direct contact
cooler 22 wherein the cracked gas stream is passed countercurrently
to water as the cooling medium to further cool the cracked gas
stream and condense the heavier hydrocarbons. Direct contact cooler
22 has an upper stage 23 and a lower stage 25. The heavier
hydrocarbons (useful as a raw gasoline product) are separated from
the cracked gas by the condensing procedure and exit direct contact
cooler 22 through line 26 together with the cooling or quenching
water which must be separated. In order to effect separation, the
quench water containing heavy hydrocarbons exiting direct contact
cooler 22 through line 26 at a temperature of about 180.degree. to
210.degree. F., thereafter, is introduced into separation drum 28
wherein the heavier hydrocarbons are separated from the quench
water. The separated hydrocarbons leave the separation drum 28
through line 30 and are recovered. The separated quench water
leaves separation drum 28 at a temperature of about 180.degree. to
210.degree. F. through line 32 and is directed into heat exchanger
34 where the quench water from separation drum 28 is cooled to a
temperature of about 155.degree. F. by indirect contact with a rich
ammonia stream 36 from the ammonia absorption refrigeration system
12. In heat exchanger 34 the rich ammonia stream is partially
vaporized by heating to a temperature of about 130.degree. to
200.degree. F. which has been found to be generally sufficient to
generate the ammonia refrigerant used for cooling the various
process streams. It is at this point where low-level waste heat is
employed as one of the heating sites to heat the rich ammonia
stream for generation of ammonia refrigerant. Thus, a rich ammonia
stream, i.e., one containing ammonia absorbed in water at a
concentration of about 70% by weight ammonia at a temperature of
about 95.degree. F. and directed from the ammonia absorption
refrigeration system 12, is introduced into heat exchanger 34
through line 36. Advantageously, in heat exchanger 34 the
temperature of the hot quench water enters at about 180.degree. to
210.degree. F. Thus, by passing the rich ammonia stream in indirect
contact with the hot quench water two purposes are achieved, i.e.,
the rich ammonia stream is partially vaporized by heating to a
temperature of from 130.degree. to 200.degree. F. and the quench
water in heat exchanger 34 is thus cooled as part of the cooling
process prior to return of the quench water to direct contact
cooler 22.
The rich ammonia stream heated to a temperature of about
130.degree. to 200.degree. F. now contains water and ammonia vapor
and leaves heat exchanger 34 through line 38 and is returned to
ammonia absorption refrigeration system 12 through line 38 where it
is converged with line 76.
Prior to returning the quench water back to direct contact cooler
22 it is preferred that the quench water be subjected to additional
cooling. Thus, quench water exiting heat exchanger 34 through line
40 at a temperature of about 155.degree. F. is directed to heat
exchanger 42 where the quench water is further cooled to a
temperature of about 130.degree. to 135.degree. F.
As will be discussed in detail, hereafter, the heated water in heat
exchanger 42 can advantageously be used as a source of heat in the
reboiler for the propane-propylene fractionation column situated in
the purification zone 10 of the ethylene process plant. The quench
water leaving heat exchanger 42 through line 44 at a temperature of
about 130.degree. to 135.degree. F. is directed back to the lower
section 25 of direct contact cooler 22 with a portion of the quench
water from line 44 being diverted through line 46 to indirect heat
exchanger 48 wherein the water is further cooled to a temperature
of about 100.degree. F. This cooled quench water is then
re-introduced into the upper stage 23 of direct contact cooler 22
through line 49.
One recognized technique utilized by the art for obtaining
substantially pure ethylene is to fractionally distill at high
pressures the ethylene process stream leaving the primary
fractionation and cooling zone 6. As best seen in FIGS. 1 and 3,
the cracked gas exiting the primary fractionation and cooling zone
6 at pressures of about 18 p.s.i.a. must be compressed to about 500
to 550 p.s.i.a. so as to ultimately obtain liquefaction and achieve
the desired product specification in the fractionation towers of
the purification zone 10. Accordingly, the uncondensed cracked gas
exiting direct contact cooler 22 through line 50 and which is in
the form of a process stream comprising, ehtylene, propylene,
ethane, butane, etc., is directed to compression zone 8 as is shown
in FIG. 1. In the compression zone 8, the cracked gas is compressed
in stages to very high pressures. This compression achieves the
high pressures necessary for distillation in the purification zone
10 as explained previously and, in addition, condenses additional
longer chain hydrocarbons. It is preferred to carry out the
compression treatment in at least two and preferably four
stages.
By passing the process stream through the compression stages the
temperatures of the compressed process stream is raised
significantly. Heretofore, as explained previously, cooling water
was utilized as the cooling medium for cooling these process
streams which was effective to reduce the temperature of the
process stream to from about 95.degree. to 105.degree. F. According
to the present invention, however, the low-level waste heat
generated by the compressors in the compression stages is utilized
to generate ammonia regrigerant in the ammonia absorption
refrigeration system 12 which ammonia refrigerant is utilized to
cool the process stream between compression stages. Advantageously,
the temperature of the process streams are reduced to a much lower
degree, i.e., 80.degree. to 85.degree. F. and as a result less
compression and, hence, smaller compressors are required in the
compression zone 8 and substantially greater amounts of the heavier
hydrocarbons can be removed prior to entering the purification zone
10. In addition, the equipment necessary to fractionally distill
the process stream in the purification zone 10 can be substantially
reduced in size over those employed utilizing cooling water as the
cooling medium in the compression zone 8. As discussed previously,
the pressure applied to the gas stream is increased in each stage.
Thus, the first stage compressor compresses the gas stream from
about 19 to 45 p.s.i.a., the second stage compressor further
compresses the gas from about 40 to 105 p.s.i.a., the third stage
compressor further compresses the gas from about 100 to 250
p.s.i.a. and the fourth stage further compresses the gas stream to
the desired final pressures of about 540 p.s.i.a. or higher.
In FIG. 3, one complete compressor stage is shown but it is to be
understood that it is merely for purposes of illustration and, in
fact, at least two and preferably four such stages are included in
the compression zone.
Thus referring to FIG. 3, it can be seen that the compression zone
8 includes an indirect heat exchanger 52 which receives the cracked
gas stream from the primary fractionation and cooling zone 6
through line 50. The cracked gas stream is cooled in heat exchanger
52 to a temperature of about 95.degree. F. using cooling water.
After cooling, the cracked gas stream exiting heat exchanger 52
through line 54 is thereafter directed to indirect heat exchanger
56 wherein the cracked gas stream is cooled to a temperature of
about 80.degree. to 85.degree. F. using an ammonia refrigerant
generated in the ammonia absorption refrigeration system 12 in a
manner which will be discussed in detail, hereafter. Heat exchanger
56 is conventional and can be of the type generally known to the
art as the shell and tube type. The ammonia refrigerant enters heat
exchanger 56 through line 58 at a temperature of about 70.degree.
F. amd at a pressure of about 130 p.s.i.a. wherein the process
stream is cooled to the desired temperature of about 80.degree. to
85.degree. F. The ammonia refrigerant exits heat exchanger 56
through line 60 at a temperature of about 70.degree. F. which at
this temperature is in the form of a vapor. The ammonia refrigerant
is returned to the ammonia absorption refrigeration system 12 which
will be discussed hereafter.
The cooled cracked gas process stream leaves heat exchanger 56
through line 62 at a temperature of about 80.degree. to 85.degree.
F. and a pressure of about 19 p.s.i.a. where it is directed to a
first compressor 64 wherein the first stage of compression of the
process stream is accomplished. Compressor 64 is conventional in
the art and can be of the centrifugal type. Associated with
compressor 64 is a steam turbine 66 which powers compressor 64.
Steam turbine 66 is driven by high pressure steam which is
introduced into turbine 66 through line 68 at temperatures of about
900.degree. F. and pressures of about 1400 p.s.i.a. According to
conventional techniques, the steam leaving the turbine was
desirably at as low a temperature and pressure as possible since
the steam's capacity for heating upon leaving the turbine was
considered too low to be economically recovered. Advantageously,
according to the present invention, the low-level waste heat
produced at this point is capable of being utilized as one of the
sources of heat for generating ammonia refrigerant for cooling the
various process streams.
Thus, the steam exiting turbine 66 at pressures of about 5 p.s.i.a.
and at temperatures of about 160.degree. F. is directed through
line 70 to heat exchanger 72 where it is passed in indirect contact
with a rich ammonia stream which enters the heat exchanger 72
through line 74. The rich ammonia stream gains heat from this
indirect contacting which heat is sufficient to generate
substantially pure ammonia in the ammonia absorption system 12.
Thus, the rich ammonia stream is introduced into heat exchanger 72
through line 74 at a temperature of about 95.degree. F. where it is
partially vaporized.
The rich ammonia stream leaves heat exchanger 72 through line 76 at
a temperature of about 130.degree. to 135.degree. F. containing
water and ammonia vapor which is to be returned to the ammonia
absorption refrigeration system 12 through line 76 where it will be
converged with line 38, best seen in FIG. 1, leaving the primary
fractionation and cooling zone 6 as will be discussed
hereafter.
The steam used as the heating medium in heat exchanger 72 is
removed through line 75.
The cracked gas process stream leaves compressor 64 through line 78
having been compressed to a pressure of about 45 to 50 p.s.i.a. and
to a temperature of about 180.degree. to 200.degree. F. and is
introduced into heat exchangers 80 and 84 where the heat generated
by compression is removed. Thus, the cracked gas stream is
introduced through line 78 to indirect heat exchanger 80 and
subjected to a first cooling where water is utilized as the cooling
medium. The cracked gas stream exits heat exchanger 80 through line
82 at a temperature of about 95.degree. to 105.degree. F. wherein
the gas stream is further cooled in second heat exchanger 84 to a
temperature of about 80.degree. to 85.degree. F. by passing it in
indirect contact with ammonia refrigerant from the ammonia
absorption refrigeration system 12. The ammonia refrigerant enters
second heat exchanger 84 through line 86 at a temperature of about
70.degree. F. and at pressures of about 130 p.s.i.a. Vaporized
ammonia refrigerant generally at temperatures of from 70.degree. F.
and pressures of about 130 p.s.i.a. leaves heat exchanger 84
through line 88 and is returned to the ammonia absorption
refrigeration system 12.
The cooled cracked gas stream exiting heat exchanger 84 through
line 90 is introduced into a separation drum 92 wherein the heavier
hydrocarbons, which have been condensed by the compression and
cooling treatments, are separated from the cracked gas process
stream. Separation can be effected by passing the cooled cracked
gas process stream into separation drum 92 wherein the heavy
hydrocarbons settle at the base of the drum and are continuously
removed through line 94 and recycled or recovered. The vapors
situated proximate the top portion of separation drum 92,
comprising the cracked gas process stream, exit through line 96
where they are directed to the second compression stage of the
compression zone.
The procedure in the compression stage outlined in detail
immediately above is repeated in each stage (preferably four) of
compression zone 8 until a final cracked gas process stream at a
pressure of about 500 to 550 p.s.i.a. is produced. In each stage of
compression, the resultant pressures are as indicated previously.
Moreover, as explained previously, in each stage heat is generated
and cooling is effected in the manner similar to the first
stage.
The cracked gas process stream having been compressed to the
desired pressures of about 500 to 550 p.s.i.a. in the last
compression stage is thereafter directed through line 96 to the
purification zone 10, best seen in FIGS. 1 and 4, for recovery of
ethylene, hydrogen, methane, propylene, propane, the C.sub.4 's,
etc., by conventional separation techniques.
The purification zone 10 generally includes the conventional
equipment and procedures for the recovery of ethylene, hydrogen,
methane, propane, propylene, butane, butylene, etc.
A demethanization section is included in the purification zone
wherein the ethylene fraction of the cracked hydrocarbon gas stream
is separated by conventional fractionation techniques from methane
and hydrogen. The method for removing methane and hydrogen from the
cracked gas stream is conventional and well known to those skilled
in the art. Merely as illustrative, the method disclosed in U.S.
Pat. No. 3,444,696 (issued May 20, 1969) can be utilized in the
demethanization section of the instant invention.
After the hydrogen and methane have been removed from the ethylene
process stream in the demethanization section, the ethylene must be
separated from the remaining hydrocarbons. One technique utilized
by the art for accomplishing ethylene separation is through the use
of a series of fractional distillation towers operating at high
pressures. Each of these towers separates constituents of the
ethylene process stream until a substantially pure ethylene product
can be recovered. The technique utilized for recovering ethylene by
a series of fractional distillation towers is also well known to
those skilled in the art. Generally, however, a series of
distillation towers is provided in which de-ethanization,
de-propanization, etc., of the ethylene process stream is
effected.
As shown in FIG. 1, the ethylene product leaves the purification
zone 10 through line 98 and is recovered. The remaining
hydrocarbons, i.e., butane, butylene, ethane, etc., are valuable
by-products of the ethylene process and leave the purification zone
through line 100 and are also recovered.
A significant fraction of hydrocarbons separated during the
recovery of ethylene in the purification zone 10 are the C.sub.3
hydrocarbons which include propane and propylene. The commercial
significance of each of these hydrocarbons particularly the
propylene makes it desirable to further separate this stream into
its two principle components. Accordingly, the propane-propylene
fraction in the purification zone 10 is directed through line 102
to a conventional fractionation column 104 which separates the
propane from the propylene by conventional fractionation
techniques.
Thus, referring to FIG. 4, the propane-propylene fraction is
introduced through line 102 into a conventional fractionation
column 104 wherein temperatures and pressures are controlled to
separate propane from propylene. The propane, being the principle
component of the bottoms liquid, is withdrawn from the
fractionation column 104 through line 106 at a temperature of about
95.degree. F. Overhead vapors, comprising principally propylene,
leave the fractionation column 104 through line 108 at a
temperature of about 78.degree. to 85.degree. F. and pressures of
about 170 to 190 p.s.i.a. and are passed through condenser 110 (a
conventional heat exchanger) where the propylene vapors are
condensed by indirect contact with an ammonia refrigerant, the
source of the ammonia refrigerant being the ammonia absorption
refrigeration system 12. Thus, an ammonia refrigerant stream is
directed from the ammonia absorption refrigeration system 12
through line 112 into condenser 110 at a temperature of about
70.degree. F. and at pressures of about 130 p.s.i.a. where it is
passed in indirect contact with the propylene vapors, effecting
condensation of the propylene vapors. The ammonia refrigerant
leaves condenser 110 through line 114 at a pressure of about 130
p.s.i.a. and at a temperature of about 70.degree. F. in the form of
a vapor. The ammonia refrigerant is then returned to the ammonia
absorption refrigeration system 12.
The condensed propylene leaving condenser 110 through line 116 at a
temperature of about 78.degree. F., is directed to recovery through
line 118 with a portion of the condensed propylene from line 116
being diverted through line 120 back to fractionation column 104
where the condensed propylene is re-introduced into the
fractionation column 104 as a reflux liquid.
In general, the pressure required to achieve the desired
propane-propylene separation in fractionation column 104 is
affected by the temperature at which the overhead vapors condenser
110 is operated. Thus, with the lower temperatures achieved in the
overhead vapors condenser 110 by the use of an ammonia refrigerant,
the fractionation column 104 can operate at lower pressures and be
reduced in size. With ammonia refrigerant available at 70.degree.
F. fractionation column 104 can operate at pressures of 170 to 190
p.s.i.a. and at temperatures of about 78.degree. to 85.degree.
F.
As discussed previously, reboiler heat for fractionation column 104
is obtained from the quench water leaving direct contact cooler 22
after the quench water has first been passed through heat exchanger
34 as best seen in FIG. 2. Thus, a liquid mixture of propane and
propylene is withdrawn as a stream from fractionation column 104
through line 122 at a temperature of about 95.degree. F. and is
introduced into heat exchanger 42 which serves as the reboiler for
fractionation column 104. The propane-propylene mixture
substantially vaporized in reboiler heat exchanger 42 is
re-introduced into fractionation column 104 through line 124.
The ammonia absorption refrigeration system 12 employed in the
present invention is of the conventional type and includes:
generation tower 126, condenser 130, storage tank 134, pressure
reducing means 142, and absorbers 148, 150, and 152. It can be seen
from FIG. 1, that the ammonia absorption refrigeration system 12
includes a generation tower 126 which generates substantially pure
ammonia from a rich ammonia stream. Thus, the rich ammonia streams
which have been partially vaporized by heating to a temperature of
about 130.degree. to 200.degree. F. by cooling various process
streams in heat exchangers 34 (FIG. 2) and 72 (FIG. 3), (including
the heat exchangers of the other stages of compression as discussed
previously but not shown) leave heat exchangers 34 and 72 through
lines 38 and 76, respectively, and are converged into line 127
where they are introduced into the lower portion of generation
tower 126. Accordingly, the distillation heat required in
generation tower 126 is supplied by the heat contained in the rich
ammonia streams which are directed through lines 38 and 76. The
operating pressure for generation tower 126 is set in relation to
the temperature of the available cooling water which is used in
overhead vapors condenser 130 so that substantially pure ammonia
will be condensed. Thus, for cooling water supplied at a
temperature of about 80.degree. F., generation tower 126 is
maintained at pressures of about 200 p.s.i.a.
Substantially pure ammonia vapors generated in generation tower 126
leave generation tower 126 through line 128 as overhead vapors at a
temperature of about 95.degree. F. and a pressure of about 200
p.s.i.a. and are introduced into condenser 130 where the ammonia
vapors are condensed by indirect contact with water as the cooling
medium. The condensed ammonia leaving condenser 130 through line
132 at a temperature of about 95.degree. F. and a pressure of about
200 p.s.i.a. is directed into storage tank 134 through line 132
with a portion of the condensed ammonia from line 132 being
diverted through line 138 back to the upper portion of generation
tower 126 where the condensed ammonia is re-introduced into
generation tower 126 as a reflux liquid.
The ammonia is contained in storage tank 134 at a pressure of about
200 p.s.i.a. and a temperature of about 95.degree. F. where it is
stored prior to being directed to the various cooling sites. Before
introducing the ammonia to the various cooling sites (i.e., heat
exchangers 56, 84, and condenser 110) it is required to reduce the
pressure of the ammonia an amount sufficient to reduce the
temperature of the ammonia to about 70.degree. F. This can be
effected by interposing pressure reducing means 142 between storage
tank 134 and the cooling sites whereby the pressure of the ammonia
can be reduced an amount sufficient to reduce the temperature of
the ammonia to about 70.degree. F. It should be understood that
although a single pressure reducing means 142 is shown in FIG. 1, a
multiplicity of pressure reducing means can be used, each of said
pressure reducing means located close to a cooling site.
Thus, ammonia liquid at a temperature of about 95.degree. F. and a
pressure of about 200 p.s.i.a. is withdrawn from storage tank 134
through line 140 and passed through a pressure reducing or
expansion valve 142 where the pressure is reduced to about 130
p.s.i.a. This is sufficient to produce an ammonia refrigerant at a
temperature of about 70.degree. F. The ammonia refrigerant leaves
pressure reducing valve 142 through line 144 and is directed
through lines 58, 86 and 112 into heat exchangers 56 and 84, and
condenser 110, respectively. After cooling in the respective heat
exchangers and condenser in the manner set forth previously, the
ammonia streams leave heat exchangers 56 and 84 and condenser 110
through lines 60, 88, 114, respectively, in the form of vapors at a
temperature of about 70.degree. F. and a pressure of about 130
p.s.i.a. and are joined into line 146 where they are introduced
into absorbers 148, 150, and 152 through lines 154, 156, and 158,
respectively.
Generation tower 126 produces lean ammonia by separating the
substantially pure ammonia vapors from the rich ammonia streams.
The lean ammonia generated in generation tower 126 leaves the tower
through line 160 in the form of a lean ammonia stream, i.e., one
containing about 66% by weight ammonia and is introduced at a
temperature of about 130.degree. F. into the first of three
absorbers 148, 150, and 152. In the absorbers, the lean ammonia
stream is enriched with ammonia to a final concentration of about
70% by weight ammonia. This is effected by contacting the lean
ammonia with substantially pure ammonia vapor which enters the
absorbers through lines 154, 156, and 158. The absorbers are
provided with cooling means (not shown) which remove the heat
generated by absorption. Thus, the lean ammonia stream enters
absorber 148 through line 160 and is withdrawn through line 162 at
a temperature of about 100.degree. F. and a concentration of about
67% by weight ammonia where it enters absorber 150 through line
162. The procedure is repeated until a final enriched ammonia
stream is provided having a concentration of about 70% by weight
ammonia and a temperature of about 95.degree. F.
The rich ammonia stream thus produced is withdrawn from absorber
152 through line 164 and is directed through lines 36 and 74 to
heat exchangers 34 (FIG. 2) and 72 (FIG. 3), respectively, in the
primary fractionation and cooling and compression zones where they
are heated by low-level waste heat as discussed previously.
It is clear that the present invention provides a system in which
low-level waste heat is efficiently utilized to generate ammonia
refrigerant which ammonia refrigerant is advantageously employed to
cool various process streams and equipment. It will, of course, be
understood that the invention can be utilized in other areas of
olefin production than those described herein. For example, in
certain instances a propylene refrigerant system is introduced in
the processing technique wherein a propylene refrigerant is
utilized as the cooling medium for condensing overhead vapors
containing primarily ethylene in the condenser associated with an
ethylene fractionation column situated in the purification zone 10.
Propylene is preferred as the cooling medium since the temperature
of available cooling water is insufficient to condense the ethylene
overhead vapors.
As is known to those skilled in the art, in a propylene refrigerant
system, compressors and turbines (similar to those used in the
compression zone 8) are utilized to compress propylene gas to the
desired operating pressures. This compression develops low-level
waste heat which can also be utilized to generate refrigerant in
the ammonia absorption refrigeration system 12. Likewise, the
ammonia refrigerant generated in the ammonia absorption
refrigeration system 12 can be utilized as a cooling medium in heat
exchangers used in the propylene refrigerant system to condense the
propylene gases.
The following Table indicates the specific pressures, temperatures,
and flow rates for the various process streams and the operating
conditions for the various equipment utilized in one illustrative
example. The information indicated in the Table is to be used in an
ethylene production process wherein 110,000 lbs/hr of ethylene are
produced and wherein a total amount of 615,500 lbs/hr. of ammonia
is circulated to process uses from the ammonia generation tower
overhead line 140.
Apparatus Parts Operating Condition and Flow Characteristcs Cracked
Gas Process Stream Flow Rate Temperature .degree. F. Pressure
p.s.i.a. C ompressors Horsepower-BHA lb/hr in out in out 64 (1st
Stage) 7650 14,890 82 186 19.2 47.6 64 (2nd Stage) 7520 14,090 82
191 42.6 105.8 64 (3rd Stage) 7740 14,640 82 194 100.7 249.9 64
(4th Stage) 8240 15,820 82 201 217.1 538.6 Propane-propylene
fractionation Temperatures Pressure column 104 95 to 78.degree. F.
170 p.s.i.a. Heat Duty Ammonia Refrigerant Overhead Vapors MM Flow
rate Temperature, .degree. F. Pressure p.s.i.a. Flow rate
Temperature .degree. F. Pressure p.s.i.a. Condensers BTUhr lb/hr in
out in out lb/hr in out in out 110 121.3 254,380 70 70 130 129.5
830,840 80 78 174 170 130 309. 627,500 95 95 203 200 Generation
Temperatures Pressure tower 126 132 to 95.degree. F. 205-200
p.s.i.a. Storage Temperature Pressure tank 134 95.degree. F. 200
p.s.i.a. Ammonia Pressure Flow rate Temperature .degree. F.
Pressure p.s.i.a. reducing lb/hr in out in out valve 142 615,500 95
70 200 130 Ammonia Vapors into Absorbers Lean Ammonia Stream into
Absorbers Flow rate Flow rate % ammonia Absorbers lb/hr Temperature
.degree. F. Pressure p.s.i.a. lb/hr by weight Temperature .degree.
F. Pressure p.s.i.a. 148 205,170 70 130 4,599,500 66 104 131.5 150
205,170 70 130 4,804,670 67 101 131 152 205,170 70 130 5,009,840 68
98 130.5 Steam Heat Rich Ammonia Stream Ammonia Refrigerant Quench
Water (low-level waste heat Cooling Water Duty flow Temp. pressure
flow Temp. pressure flow Temp. pressure flow Temp. pressure flow
Temp. pressure MM rate .degree. F. p.s.i.a. rate .degree. F.
p.s.i.a. rate .degree. F. p.si.a. rate.degree. F. p.s.i.a. rate
.degree. F. p.si.a. Heat Exchanger BUT/hr lb/hr in out inout lb/hr
in out in out lb/hr in out in out lb/hr in out in out lb/hr in out
in out 34 153.6 95 132 207 205 185 156 20 19 5.39 42 121.2 .times.
10.sup.6 156 134 19 18 48 134 100 18 17 80 120 0.68 52 13.56
.times. 10.sup.6 80 100 56 15.55 32553 70 70 130 129.5 one turbine
drives 72 all four stages) 208.7 95 132 207 205 160 160 4.7 4.5
0.58 80 (1st Stage) 23.0 .times. 10.sup.6 80 120 0.53 80 (2nd
Stage) 21.3 .times. 10.sup.6 80 120 0.59 80 (3rd Stage) 23.6
.times. 10.sup.6 80 120 0.82 80 (4th Stage) 32.8 .times. 10.sup.6
80 120 84 (1st Stage) 5.0 10,467 70 70 130 129.5 84 (2nd Stage) 4.2
8,792 70 70 130 129.5 84 (3rd Stage) 3.3 6.908 70 70 130 129.5
While the foregoing preferred embodiment has been described with
respect to an ammonia evaporation pressure of 130 p.s.i.a. which
corresponds to a temperature of about 70.degree. F. as the ammonia
refrigerant is passed through the various process heat exchangers,
ammonia refrigerant at lower temperature levels may also be
obtained without departing from the process scheme described in the
foregoing. Hence, without departing from the process scheme of the
invention, the ammonia refrigerant in line 140 may be flashed
through valve 142 to a pressure as low as about 100 p.s.i.a. which
will result in a refrigeration temperature of about 55.degree. F.
as the ammonia is passed to the various heat exchanger uses
associated with line 144.
When the ammonia refrigeration at a temperature level of about
55.degree. F. is so obtained by the process of the invention, the
ammonia refrigerant to condenser 110 through line 112 would
therefore also be available at a temperature of 55.degree. F. Thus,
with ammonia refrigerant available at a level of 55.degree. F. to
condenser 110, propane-propylene fractionation column 104 can be
operated at a pressure as low as about 140 p.s.i.a. At 140 p.s.i.a.
the propylene rich overhead from fractionation column 104 will be
at a temperature of about 63.degree. F. as it is passed through
line 108 to condenser 110. Operation of propane-propylene
fractionation column 104 at lowered pressures is desirable since
propylene and propane are more easily separated as the
fractionation pressure is lowered. Thus, the lower pressure
operation of fractionation column 104 permitted by use of ammonia
refrigerant at temperatures between 55.degree. to 70.degree. F. as
contemplated by the process of this invention results in an easier
separation between propylene and propane and thus fewer
fractionation trays are required in column 104.
Additionally, it should be understood that with ammonia refrigerant
levels lower than 70.degree. F., for example between 55.degree. F.
and 70.degree. F. the cracked gas streams passing through heat
exchangers 56 and the first, second and third compressor stage heat
exchangers 84 can be cooled to temperatures lower than 80.degree.
to 85.degree. F. Specifically, when the ammonia refrigerant level
is 55.degree. F. the cracked gas passing through heat exchangers 56
and 84 can be readily cooled to approximately 65.degree. F. The
lower cracked gas temperatures at the inlet to the compressor
stages 64 (stages one through four) results in reduced horsepower
requirements at each stage of compression.
Although the embodiment described herein is in terms of a single
generation tower and absorbers in series it should be understood
that several generation tower in parallel each heated by a separate
source, as well as several additional absorbers in parallel, may be
employed.
While we have fully described an embodiment of the foregoing
invention, it is to be understood that this description is offered
by way of illustration only. The range of adaptability of the
process presented herein is contemplated to include many variations
and adaptions of the subject matter within the scope of olefin
production, and it is to be understood that this invention is to be
limited only by the scope of the appended claims.
* * * * *